This invention relates to an electric motor unit capable of high-speed, high-accuracy rotation of a motor shaft, more specifically to an electric motor unit for high-speed, high-accuracy rotation of a polygonal mirror in a laser printer, for example.
Recently, printers using lasers have been developed. One such prior art laser printer comprises, for example, a semiconductor laser 1, a polygonal mirror 2, and a drum 4 having a photoconductor surface 3, as shown in FIG. 1. In this laser printer, a laser beam generated from the semiconductor laser 1 is converged by a beam compressor 5 and directed toward a modulator 6. In the modulator 6, the intensity of the laser beam is modulated in response to an electrical signal. After passing the modulator 6, the laser beam is projected on the polygonal mirror 2 through a beam expander 7. Reflected by a reflecting surface of the polygonal mirror 2, the laser beam is then projected on the photoconductor surface 3 of the drum 4 through a projection lens 9. Since the polygonal mirror 2 is rotated at a constant speed, the laser beam reflected by the reflecting surface of the polygonal mirror 2 is deflected so that the photoconductor surface 3 is scanned with the laser beam. Since the laser beam is intensity-modulated by the modulator 6, and the drum 4 is rotated in a specified direction at a constant speed, a latent image, such as a pattern or characters, is formed on the photoconductor surface 3.
In the aforementioned laser printer, the polygonal mirror 2 is rotated at a high and constant speed with high accuracy by a motor unit 8. In order to provide this kind of rotation of the polygonal mirror 2, the motor unit 8 must have the following properties:
(i) No contamination of the polygonal mirror by spindle oil or the like during rotation. PA1 (ii) Long life. PA1 (iii) Less friction torque loss at the start or during steady rotation. PA1 (iv) Ease of transport. PA1 (v) Ease of balancing. PA1 (vi) No rotating members having a complicated vibration mode, not undergoing a high-frequency vibration.
In a conventional motor unit which meets these requirements, the spindle is rotatably supported by a bearing of hydro dynamic-pressure type. Dynamic-pressure type bearings include bearings of herringbone and tilting-pad types. The bearings of both these types, however, are not easy to manufacture.
The use of a non-controlled, repulsion-type permanent magnet bearing as the thrust bearing for the spindle is disclosed in Japanese Patent Publication No. 6854/78. However, it is hard for this magnetic bearing to balance itself in a static manner. If an external dynamic force is applied to the magnetic bearing, the rigidity of the bearing varies with the direction of the unfixed vector of the working force. Thus, the magnetic bearing is poor in both stability and in rigidity. As compared with the non-controlled magnetic thrust bearing, a controlled magnetic thrust bearing combining a permanent magnet and a coil has the advantages of good stability and high rigidity. Because it requires peripheral equipment, however, the controlled bearing makes the motor complicated in construction. Thus, it is difficult to manufacture such a motor at low cost.
An induction motor or DC motor may also be used for the motor.
However, the induction motor is provided with many accessories, and so becomes expensive, and it also produces great heat. And the DC motor cannot easily maintain a steady speed.